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Abstract:

A system for balancing charge within a battery pack with a plurality of
cells connected in series, including a capacitor; a processor configured
to select a combination of donor cells and receiver cells from the
plurality of cells in one of the following two modes: (1) a first mode
where the number of donor cells is equal to the number of receiver cells,
and (2) a second mode where the number of donor cells is greater than the
number of receiver cells; and a plurality of switches that electrically
connect the capacitor to the donor cells to charge the capacitor, and
that electrically connected the capacitor to the receiver cells to
discharge the capacitor. The transfer of charge between cells in the
plurality of cells through the capacitor balances the charge within the
battery pack.

Claims:

1. A system for balancing charge within a battery pack with a plurality of
cells connected in series, comprising:a capacitor;a processor configured
to select a combination of donor cells and receiver cells from the
plurality of cells in one of the following two modes: (1) a first mode
where the number of donor cells is equal to the number of receiver cells,
and (2) a second mode where the number of donor cells is greater than the
number of receiver cells;a plurality of switches that electrically
connect the capacitor to the donor cells to charge the capacitor, and
that electrically connected the capacitor to the receiver cells to
discharge the capacitor;wherein the transfer of charge between cells in
the plurality of cells through the capacitor balances the charge within
the battery pack.

2. The system of claim 1, further comprising a sensor coupled to each of
the cells that senses or determines the charge of each cell, and wherein
the processor is configured to select a combination of donor cells and
receiver cells based on the sensed charge.

3. The system of claim 2, wherein the sensor is a voltage sensor.

4. The system of claim 1, wherein the switches are transistors.

5. The system of claim 4, wherein the switches are each field effect
transistors.

6. The system of claim 1, wherein the switches include a first set of
switches, each with one end electrically connected to the negative
terminal of a cell and another end electrically connected to one terminal
of the capacitor, and a second set of switches, each with one end
electrically connected to the positive terminal of a cell and another end
electrically connected to the other terminal of the capacitor, wherein an
actuated switch from the first set of switches and an actuated switch
from the second set of switches electrically connects the cells in
between the actuated switches in parallel with the capacitor.

7. The system of claim 6, wherein each of the switches in the first set of
switches and the second set of switches are individually controlled.

8. The system of claim 1, wherein the switches includes a multiplexer.

9. The system of claim 8, wherein the switches includes a first
multiplexer electrically connected to the positive terminal of each of
the cells and to one terminal of the capacitor, and a second multiplexer
electrically connected to the negative terminal of each of the cells and
to the other terminal of the capacitor.

10. The system of claim 1, wherein the processor is configured to select a
combination of donor cells and receiver cells from a first portion of the
plurality of cells, and further comprising:a second capacitor;wherein the
processor is further configured to select a second combination of donor
cells and receiver cells from a second portion of the plurality of cells
in one of the following two modes: (1) a first mode where the number of
donor cells is equal to the number of receiver cells, and (2) a second
mode where the number of donor cells is greater than the number of
receiver cells;and further comprising:a second plurality of switches that
electrically connects the second capacitor to the donor cells of the
second combination to charge the second capacitor, and that electrically
connects the capacitor to the receiver cells of the second combination to
discharge the second capacitor;wherein the transfer of charge between
cells in the plurality of cells through the second capacitor balances the
charge within the battery pack.

11. The system of claim 10, wherein a cell of the plurality of cells is a
member of both the first portion and second portion of the plurality of
cells.

12. The system of claim 10, where charge is transferred between the first
and second portions of the plurality of cells.

13. The system of claim 10, wherein the processor includes a first
processor that is configured to select a combination of donor cells and
receiver cells from the first portion of the plurality of cells and a
second processor that is configured to select a combination of donor
cells and receiver cells from the second portion of the plurality of
cells.

14. A method for balancing charge within a battery pack with a plurality
of cells connected in series, comprising the steps of:selecting a
combination of donor cells and receiver cells from the plurality of cells
in one of the following two modes: (1) a first mode where the number of
donor cells is equal to the number of receiver cells, and (2) a second
mode where the number of donor cells is greater than the number of
receiver cells;electrically connecting a capacitor to the donor cells to
charge the capacitor; andelectrically connecting the capacitor to the
receiver cells to discharge the capacitor;wherein the steps of charging
and discharging the capacitor moves charge within the battery pack
through the capacitor and balances the charge in the battery pack.

15. The method of claim 14, further comprising sensing the charge in each
of the cells, and wherein the step of selecting a combination of donor
cells and receiver cells includes selecting a combination of donor cells
and receiver cells based on the sensed charge.

16. The method of claim 14, wherein the step of selecting a combination of
donor cells and receiver cells includes the steps of selecting a cell
having a greater voltage potential than the capacitor for the donor cell
and selecting a cell having a lower voltage potential than the charged
capacitor for a receiver cell.

17. The method of claim 14, wherein the step of selecting a combination of
donor cells and receiver cells includes the steps of selecting cells
that--if connected in a series--would have a combined greater voltage
potential than the capacitor for the donor cells, and connected the
selected cells in a series.

18. The method of claim 14, wherein the step of selecting a combination of
donor cells and receiver cells includes selecting a combination that is
optimized relative to the resulting charge transfer rate between cells,
the resulting charge transfer efficiency between cells, and the resulting
charge balance of the battery pack.

19. The method of claim 17, wherein the step of selecting a combination
includes running a quadratic program that determines a combination that
is optimized relative to the resulting charge transfer rate between
cells, the resulting charge transfer efficiency between cells, an the
resulting charge balance of the battery pack.

20. The method of claim 14, wherein the step of selecting a combination of
donor cells and receiver cells includes selecting a combination according
to the first mode to optimize for high resulting charge transfer
efficiency.

21. The method of claim 14, wherein the step of selecting a combination of
donor cells and receiver cells includes selecting a combination according
to the second mode to optimize for fast resulting charge transfer rate.

22. The method of claim 14, wherein the step of selecting a combination of
donor cells and receiver cells from the plurality of cells includes
selecting a combination from a set of available combinations of donor
cells and receiver cells.

23. The method of claim 22, wherein the step of selecting a combination
from a set of available combinations of donor cells and receiver cells
includes the steps of assigning a score to each available combination
based on the resulting charge transfer rate between cells, resulting
charge transfer efficiency between cells, and resulting charge balance of
the battery pack, and selecting a combination based on the score.

24. The method of claim 23, wherein the step of assigning a score includes
adjusting the score based on the usage scenario.

25. The method of claim 24, wherein the step of adjusting the score
includes adjusting the score based on the rate of energy transfer to and
from the battery pack.

26. The method of claim 24, wherein the step of adjusting the score
includes adjusting the score based on a user preference.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of U.S. Provisional Application
No. XX/XXX,XXX filed on 23 Mar. 2009 and entitled "Multiple Cell Charge
Pump for Active Battery Management" (and naming Jim Castelaz, Jessica
Riley, Vishal Parikh, and Steven Diamond as the inventors), which is
incorporated in its entirety by this reference.

TECHNICAL FIELD

[0002]This invention relates generally to the portable electric power
field, and more specifically to a new and useful charge balancing system
and method in the battery pack management field.

BACKGROUND AND SUMMARY

[0003]Battery packs are increasingly produced with many battery cells that
are electrically connected to each other within the battery pack. While
of the same specification, each battery cell within battery packs may
operate differently; in particular, each battery cell may hold charge
differently. This may be a result of manufacturing differences between
cells, age difference between cells, or any other suitable source of
differences. A battery pack with cells that are at different charge
levels may have a decreased battery pack lifetime. For example, a cell
within a battery pack that has a higher charge level may operate at a
temperature that is higher than an optimal operating temperature for the
cell. This may cause that particular cell within the battery pack to
catastrophically fail, which may then lead to neighboring cells
catastrophically failing and/or may lead to failure of the battery pack.
This is especially true when the rate of energy transfer to and from the
battery pack is substantially high (for example, during high power charge
or discharge situations). If there is a charge imbalance within the
battery pack, a high rate of energy transfer to and from the cells may
cause the charge imbalance to be further amplified in a substantially
short period of time, which may lead to increased chance of failure of
the battery pack.

[0004]Currently available systems and methods for balancing charge include
dissipating extra charge from imbalanced cells, which results in the
waste of the extra charge through the resistors. Other available systems
balance charge by transferring charge from one cell to another. Available
charge balancing circuits are complicated and expensive to manufacture
(e.g., charge balancing circuits that require sensors and capacitors at
each cell within the battery pack). Other available charge balancing
circuits may be too slow in balancing charge within the battery pack
(e.g., charge balancing circuits that transfer charge between imbalanced
cells by utilizing the difference in voltage potential between the
imbalanced cells, which may be very slow if difference is relatively
small, and/or may only allow for charge transfer between certain cells
within the battery pack). As mentioned above, charge imbalances may be
amplified in a substantially short period of time in scenarios where the
rate of energy transfer to and from the battery pack is high. If the
charge balancing circuit is not fast enough to balance charge to prevent
the amplification of charge imbalance, battery pack failure may not be
prevented.

[0005]Thus, there is a need in the battery pack management field to create
a new and useful charge balancing system and method that is relatively
simple, cost effective, fast, and flexible. This invention provides such
a new and useful charge balancing system and method.

[0006]The system of the preferred embodiments for balancing charge within
a battery pack with a plurality of cells connected in series includes a
capacitor, a processor that is configured to select a combination of
donor cells and receiver cells from the plurality of cells in one of the
following two modes: a first mode where the number of donor cells is
equal to the number of receiver cells and a second mode where the number
of donor cells is greater than the number of receiver cells, and a
plurality of switches that electrically couple the capacitor to the donor
cells to charge the capacitor, and electrically couple the capacitor to
the receiver cells to discharge the capacitor. The charge balancing
system may also include a sensor coupled to each of the plurality of cell
that senses or determines the charge of each cell. In this variation, the
processor is configured to utilize the sensed charge to select a
combination of donor cells and receiver cells. In the preferred
embodiments, charge is moved between cells of the battery pack through
the charge and discharge of the capacitor, and the movement of the charge
between the donor cells and the receiver cells balances the charge within
the battery pack.

[0007]In existing prior art, such as U.S. Pat. No. 6,518,725, charge is
moved from a cell with a higher voltage potential to a cell with a lower
voltage potential through a capacitor. The initial charge/discharge rate
(or charge/discharge current) of the capacitor is directly related to
both the time constant (which is determined by the capacitance of the
capacitor and the total resistance within the circuit) and the difference
in voltage potential between the capacitor and the cell that
charges/discharges the capacitor. For any set time constant, the speed of
cell balancing circuits that moves charge from one cell to another is
limited by the maximum voltage potential difference between the two
cells. In most cases, especially for cells whose state of charge is
neither very high nor very low, the voltage potential difference between
two imbalanced cells may not be very large, further slowing the charge
transfer rate. The resulting charge transfer rate in such charge
balancing circuits may not be fast enough for certain usage scenarios.
For example, an increased rate of energy transfer into or out of the
battery pack during high power charging or discharging may amplify
existing charge imbalances in a very short period of time, which may lead
to catastrophic failure of the battery pack. In a more specific example,
a particular cell within the battery pack may charge at a rate that
causes its voltage to increase at an average of 0.5 volts per hour faster
than other cells in the battery pack. A charge balancing circuit that is
slow (for example, capable of transferring only enough charge away from
the imbalanced cell and into other cells in the battery pack to decrease
the voltage of the imbalanced cell by 0.1 volts per hour) will not be
fast enough to prevent the imbalanced cell from becoming more imbalanced
and possibly failing.

[0008]In the system of the preferred embodiments, the processor may select
a combination of donor cells and receiver cells in a first mode where the
number of donor cells and receiver cells are equal and in a second mode
where the number of donor cells is greater than the number of receiver
cells. In usage scenarios that require a faster speed of charge
balancing, the processor may select a combination of donor cells and
receiver cells according to the second mode. For example, a substantially
large number of donor cells that are connected in series (for example, if
the number of the plurality of cells is N, then the number of donor cells
may be up to N cells) to charge the capacitor and a substantially small
number of receiver cells that are connected in series (for example, one)
to discharge the capacitor. Thus, the voltage potential difference
between the donor cells connected in series and the capacitor is
significantly high, increasing the initial charge rate of the capacitor.
The charged capacitor is then at a substantially higher voltage potential
than the receiver cell, increasing the initial discharge rate of the
capacitor and substantially increasing the charge transfer rate between
the cells within the battery pack. Additionally, the increased combined
voltage potential of the donor cells allows for an increased amount of
charge to be transferred to the receiver cells in a fixed-time charge and
discharge cycle of the capacitor, increasing the speed of charge
balancing within the battery pack over existing charge balancing circuits
by orders of magnitude. The processor may alternatively select any other
suitable combination of donor cells and receiver cells to increase the
charge transfer rate between cells.

[0009]With increased rate of charge transfer between the donor cells, the
capacitor, and the receiver cells, there may be a decrease in charge
transfer efficiency between cells. For example, with the increased charge
current, energy may be lost through heat dissipated through the circuit.
Thus, in usage scenarios that do not require a high rate of charge
transfer between cells, the processor may select a combination of donor
cells and receiver cells according to the first mode. For example, one
donor cell and one receiver cell. This will result in a lower initial
charge/discharge rate of the capacitor, which may allow for an increase
in charge transfer efficiency between cells. Alternatively, the processor
may select a combination of donor cells and receiver cells according to
the second mode, but with a smaller difference between the number of
donor and number of receiver cells. The processor may alternatively
select any other suitable combination of donor cells and receiver cells
to increase the charge transfer efficiency between cells.

[0010]The charge balancing system of the preferred embodiments allows for
increased flexibility in charge balancing. By allowing selection of
combinations of donor cells and receiver cells of a first and a second
mode, any number of and any of the plurality of cells may function as
donor cells and receiver cells interchangeably and the rate of charge
transfer and the efficiency of charge transfer between cells may be
optimized for different usage scenarios, which may result in a more
balanced and healthy battery pack.

BRIEF DESCRIPTION OF THE FIGURES

[0011]FIG. 1 is a schematic representation of the charge balancing system
of the preferred embodiments.

[0012]FIGS. 2a and 2b are schematic representations of the movement of
charge between the plurality of cells when charging the capacitor and
discharging the capacitor, respectively.

[0013]FIG. 2c is a graphical representation of the charge within the
capacitor during charge and discharge cycles.

[0014]FIG. 3 is a schematic representation of the charge balancing system
of the preferred embodiments with a second variation of the plurality of
switches.

[0015]FIG. 4 is a graphical representation of the balance of charge within
the plurality of cells.

[0016]FIGS. 5 and 6 are schematic representations of variations of the
charge balancing system with more than one charge balancing circuit.

[0017]FIG. 7 is a schematic representation of the charge balancing method
of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]The following description of the preferred embodiments of the
invention is not intended to limit the invention to these preferred
embodiments, but rather to enable any person skilled in the art to make
and use this invention.

1. System for Balancing Charge

[0019]As shown in FIGS. 1 and 2, the system 100 of the preferred
embodiments for balancing charge within a battery pack with a plurality
of cells 110 connected in series includes a capacitor 120, a processor
130 that is configured to select a combination of donor cells 114 and
receiver cells 116 from the plurality of cells 110 in one of following
two modes: a first mode where the number of donor cells is equal to the
number of receiver cells and a second mode where the number of donor
cells is greater than the number of receiver cells, and a plurality of
switches 140 that electrically couple the capacitor 120 to the donor
cells 114 to charge the capacitor 120 and electrically couple the
capacitor to the receiver cells 116 to discharge the capacitor 120. The
charge balancing system 100 may also include a sensor 112 coupled to each
of the plurality of cells 110 that senses or determines the charge of
each cell and the processor 130 is configured to utilize the sensed
charge to select a combination of donor cells 114 and receiver cells 116.
In the preferred embodiments, charge is moved between cells 110 of the
battery pack through the charge and discharge of the capacitor 120, and
the movement of the charge between the donor cells 114 and the receiver
cells 116 preferably balances the charge within the battery pack.

[0020]The charge balancing system 100 of the preferred embodiments is
preferably applied to a battery pack with a plurality of cells 110 that
are connected in series. As shown in FIGS. 1 and 2, the plurality of
cells 110 may include eight cells 110 that are connected in series, but
may alternatively include any other suitable number of cells connected in
series. Each cell in the battery pack may be connected in series.
Alternatively, a portion of the cells in the battery pack may be
connected in series; for example, the cells in the battery pack may be
arranged in a combination of series and parallel electrical connections.
In this variation, the charge balancing system 100 is preferably applied
to the portion of cells in the battery pack that is connected in series.
However, the charge balancing system 100 may be applied to any other
suitable combination of cells within the battery pack. Each of the
plurality of cells 110 is preferably a unitary energy storage unit, but
may alternatively include multiple energy storage units that are
connected to each other in series, parallel, or any other suitable
combination of series and parallel electrical connections to form a cell
110. For example, each cell 110 may be a group of individual energy
storage units that are connected in parallel and the charge balancing
system 100 preferably balances charge among each group of energy storage
units. However, the plurality of cells 110 may be of any other suitable
type of arrangement. The battery pack may be used for any suitable
electrical power application, for example, a portable computer, a mobile
phone, a grid-connected battery backup system, an electric vehicle, or a
hybrid-electric vehicle. However, the charge balancing system 100 may be
applied to any other suitable type of battery pack.

[0021]As shown in FIGS. 2a, 2b, and 2c, the plurality of switches 140 of
the preferred embodiments couples the capacitor 120 to at least one donor
cell 114 (two donor cells 114(6) and 114(7) are shown in FIGS. 2a and 2b)
to charge the capacitor 120, and then couples the capacitor 120 to at
least one receiver cell 116 (one receiver cell 116(4) is shown in FIGS.
2a and 3b) to discharge the capacitor 120. As shown in FIG. 2c, the
charge and discharge cycle of the capacitor 120 is preferably repeated,
The processor may also select a second combination of donor cells 114 and
receiver cells 116 and the charge balancing cycle may be repeated with
the second combination. This repetitive process may continue until all of
the cells 110 are substantially equally charged. The processor 130
preferably actuates the plurality of switches 140, but the plurality of
switches 140 may alternatively be coupled to a second processor that
communicates with the processor 130 to receive communication on the
combination of the donor cells 114 and receiver cells 116 to connect to
the capacitor using the plurality of switches 140.

[0022]The plurality of switches 140 is preferably arranged to connect any
single cell 110 or any combination of any contiguous cells 110 to the
capacitor 120 as donor cells 114 and receiver cells 116. The plurality of
switches 140 is preferably capable of coupling each cell as a donor cell
114 and receiver cell 116 interchangeably, depending on the combination
of donor cells 114 and receiver cells 116 selected by the processor. As
shown in FIG. 1, a first variation of the plurality of switches 140
includes a first set of switches 142, each with one end electrically
connected to the negative terminal of a cell 110 and another end
electrically connected to one terminal of the capacitor 120. The
plurality of switches 140 also includes a second set of switches 144,
each with one end electrically connected to the positive terminal of a
cell 110 and another end electrically connected to the other terminal of
the capacitor 120. As shown in FIGS. 2a and 2b, when a switch from the
first set of switches 142 coupled to the negative terminal of a cell
(here shown as cell number 114(6)) is actuated and a switch from the
second set of switches 144 coupled to the positive terminal of another
cell (here shown as cell number 114(7)) is actuated, the cells 110 in
between the actuated switches are then electrically connected in parallel
with the capacitor 120 (here shown as donor cells 114). Similarly, when a
switch of the first set of switches 142 coupled to the negative terminal
of a cell (here shown as cell number 114(4)) is connected and a switch of
the second set of switches 144 coupled to the positive terminal of the
same cell is connected, only that one cell is coupled electrically
connected in parallel with the capacitor 120 (here shown as a receiver
cell 116). This arrangement of the plurality of switches 140 allows for
any number of and any selection of the plurality of cells 110 to be
electrically connected to the capacitor 120.

[0023]Each switch within the first and second sets of switches 142 and 144
are preferably individually controlled to establish the desired
electrical connection with the capacitor 120 with the donor cells 114 and
the receiver cells 116. The number of switches in each of the first set
of switches 142 and the second set of switches 144 is preferably the same
number of the plurality of cells 110. However, the number of switches in
each set of switches 142 and 144 may be less than the number of the
plurality of cells 110, for example, two cells may be treated as a group
that are always concurrently connected to the capacitor 120. However,
there may be any other suitable number of switches in the first and
second set of switches 142 and 144. Each of the switches 140 in this
variation may be a transistor, for example, a field effect transistor
(FET) such as a metal oxide semiconductor field effect transistor
(MOSFET), or a bipolar transistor. The switches 140 is preferably able to
stand off the voltage of the plurality of cells 110 connected in series,
in other words, when not actuated, each of the switches preferably does
not allow flow of current from the plurality of cells 110. However, the
switches 140 in this variation may be any other suitable type of switch.

[0024]In a second variation, as shown in FIG. 3, the plurality of switches
140 may include a multiplexer 146. The multiplexer 146 functions to
simplify the signals necessary to actuate the plurality of switches 140
to couple the desired donor cells 114 and receiver cells 116 to the
capacitor 120. By simplifying the signals necessary to actuate the
plurality of switches 140, fewer control signals are necessary and
computational power required may be less, which may decrease the cost of
the charge balancing circuit. The multiplexer 146 may function to replace
one of the first set of switches 142 or the second set of switches 144 of
the first variation and preferably couples the plurality of cells 110 to
the capacitor 120 in a similar or identical manner as in the first
variation. The plurality of switches 140 may include a first and second
multiplexer 146 and 148, where the first multiplexer 146 replaces the
first set of switches 142 and the second multiplexer 148 replaces the
second set of switches 142. Both the first and second multiplexers 146
and 148 preferably function to couple the plurality of cells 110 to the
capacitor 120 in a similar or identical manner as the first and second
set of switches 142 and 144, respectively, of the first variation. In
this variation, the number of cells in the plurality of cells 110 is
preferably of a power of two, which allows more efficient use of the
multiplexers as the plurality of switches 140. However, the multiplexer
of the second variation may be of any other suitable type. Additionally,
any other suitable component may be used to simplify the signals
necessary to actuate the plurality of switches 140 to couple the desired
donor cells 114 and receiver cells 116 to the capacitor 120.

[0025]As mentioned above, in the variations with more than one donor cell
114 and/or more than one receiver cell 116, the plurality of switches 140
preferably also couples each donor cell 114 to each other in a series
connection and each receiver cell 116 to each other in a series
connection. However, the plurality of switches 140 may alternatively
couple each donor and receiver cell 114 and 116 in any other suitable
arrangement, for example, the plurality of switches 140 may couple each
receiver cell 116 to each other in a parallel connection to maintain a
substantially low combined voltage potential of the receiver cells 116.
However, the plurality of switches 140 may be of any other suitable
arrangement.

[0026]The capacitor 120 of the preferred embodiments functions to accept a
charge from the donor cells and discharge a charge to the receiver cells.
The capacitance of the capacitor 120 preferably holds a substantial
amount of charge (such as 33 milli-Farads), which may decrease the time
needed to balance charge between cells. The capacitor 120 is preferably
of a non-variable type and preferably has substantially low charge
leakage to increase the efficiency of charge transfer between cells. The
capacitor 120 is preferably of a substantially small size to allow
integration into the battery pack. However, the capacitor 120 may be of
any other suitable type of capacitor.

[0027]The charge and discharge cycle of the capacitor during charge
balancing is preferably based on the capacitance and the total resistance
of the circuit. The total resistance of the circuit may substantially
result from the resistance in the plurality of switches 140 (the
individual switches in the first variation and the multiplexers in the
second variation). The charge and discharge times of the capacitor 120
during cell balancing (in other words, the time that the plurality of
switches electrically couples the donor cells 114 or receiver cells 116,
respectively, to the capacitor 120) is preferably fixed. In this
variation, the amount of charge transferred in one charge and discharge
cycle depends on the difference in voltage potential between the donor
cells 114 and the receiver cells 116. The charge time selected preferably
allows for the capacitor 120 to charge to a voltage potential that is
above the voltage potential of the receiver cell 116 and the discharge
time selected preferably allows a substantial amount of charge from the
charged capacitor 120 to transfer to the receiver cell 116. The discharge
time may be selected to allow a maximum amount of charge to be
transferred to the receiver cell 116, which may shorten the time required
to balance charge amongst the cells 110. Alternatively, the charge and
discharge times of the capacitor 120 may be adjusted based on the voltage
potential difference of each selected combination of donor cells 114 and
receiver cells 116. However, the charge and discharge times may be
selected using any other suitable method.

[0028]In the variation of the charge balancing system that includes a
sensor 112 that is coupled to each of the plurality of cells 110, the
sensor 112 preferably includes a voltage sensor that senses the voltage
potential within each cell 110. The detected voltage may then be used to
derive the estimated state of charge of the cell, based on the cell's
internal chemistry-dependent relationship between voltage and state of
charge. Alternatively, the sensor 112 may include a current sensor that
detects the current going through each cell 110. The detected current may
then be integrated to derive the amount of charge that is contained
within each cell 110. The sensor may also include a voltage sensor and a
current sensor that cooperate to provide a more accurate measure of each
cell's state of charge. However, any other suitable type of sensor may be
used.

[0029]As shown in FIG. 3, the charge balancing system 100 of the preferred
embodiments may also include digital isolators 150 that function to allow
the plurality of switches 140 to be controlled by control signals that
may be referenced to voltages that are different from the voltage
potentials within the battery pack and/or the charge balancing circuit.
The digital isolators 150 may be opto-isolators, magnetic isolators, or
any other suitable type of digital isolator.

[0030]The processor 130 of the preferred embodiments functions to select a
combination of donor cells and receiver cells from the plurality of
cells. As mentioned above, the processor 130 may select a combination of
any number of and any selection of cells as donor cells 114 and receiver
cells 116 from the plurality of cells 110. The processor 130 preferably
selects a cell with a higher voltage potential than that of the capacitor
120 as a donor cell 114 to charge the capacitor and preferably selects a
cell with a lower voltage potential than the charged capacitor 120 as the
receiver cell 116 to discharge the capacitor. Alternatively, the
processor may select cells that are connected in series that have a
combined voltage potential that is higher than that of the capacitor 120
as donor cells 114. In this variation, each of the donor cells 114 may
have a voltage potential that is higher than that of the capacitor 120.
Alternatively, in this variation, a donor cell 114 may have a voltage
potential that is lower than that of the capacitor 120. However, the
donor cells 114 and receiver cells 116 may be of any other suitable
voltage potential relative to the capacitor 120.

[0031]In a first variation, the processor 130 selects the combination of
donor cells 114 and receiver cells 116 based on the charge state of each
cell 110. In this variation, the processor 130 preferably selects a donor
cell 114 of a higher voltage potential than the capacitor to charge the
capacitor 120 and a receiver cell 116 of a lower voltage potential than
the charged capacitor 120. In this variation, the processor 130 may
evaluate the charge of each cell 110 that is sensed by the sensor 112 and
selects the cells 110 with the highest charge to be donor cells and cells
114 with the lowest charge to be receiver cells 116. Alternatively, the
processor 130 may determine a desired charge level for each of the
plurality of cells 110 and when any cell is detected to have a charge
level higher than the desired charge level of that cell, the processor
130 selects that particular cell as a donor cell 114 and searches for a
cell that has a charge level that is lower than (or substantially equal
to) the desired charge level of that cell and selects that cell as a
receiver cell 114. In this variation, the desired charge level of each
cell may be substantially equal. The processor 130 may select a
combination of donor cells 114 and receiver cells 116 in either of the
first or second modes. If increased efficiency of charge transfer is
desired, the processor may select a combination according to the first
mode. If increased rate of charge transfer is desired, the processor 130
may select a combination according to the second mode. However, the
processor 130 may select donor cells 114 and receiver cells 116 based on
the charge state using any other suitable method.

[0032]In a second variation, the processor 130 selects the combination of
donor cells 114 and receiver cells 116 by selecting a combination that is
optimized relative to a characteristic selected from the resulting charge
transfer rate between cells, the resulting charge transfer efficiency
between cells, and the resulting charge balance of the battery pack.
Because it may be difficult to determine a combination that optimizes for
more than one of the above characteristics (for example, it may be
difficult to find a combination that optimizes for both the charge
transfer rate and the charge transfer efficiency because more energy is
lost when the charge transfer rate is substantially high), the processor
130 may select a combination that optimizes relative to only one
characteristic. In a first example, the processor is configured to select
a combination of donor cells 114 and receiver cells 116 according to the
first mode to optimize for high resulting charge transfer efficiency
between cells. In a second example, the processor is configured to select
a combination of donor cells 114 and receiver cells 116 according to the
second mode to optimize for high resulting charge transfer rate between
cells. However, the processor 130 may select any other suitable
combination that optimizes for only one characteristic.

[0033]The processor 130 may also select a combination that substantially
optimizes for more than one characteristic. In a first example, the
processor 130 may select a combination that optimizes one characteristic
within limits for the other characteristics; for example, the processor
130 may select a combination that optimizes for the highest charge
transfer efficiency that can be achieved while bringing all cells to
within 1% of any other cell's state of charge within 4 hours. In a second
example, the processor 130 may select a combination that optimizes
performance measured by a metric that is the combination of multiple
characteristics; for example, the processor 130 may select a combination
that minimizes the weighted sum of the total charge lost and the standard
deviation of the charge within each of the plurality of cells over a
period of time. This type of optimization may result in a selection of a
combination of donor cells 114 and receiver cells 116 according to either
the first or the second mode. To select an optimized combination of donor
cells 114 and receiver cells 116, the processor 130 may run optimization
calculations. The optimization calculations are preferably based on the
sensed charge within each cell 110. For example, the processor 130 may
run a quadratic program that determines a set of donor and receiver cells
in the plurality of cells 110 where, in the process of moving charge
between cells, both rate of charge transfer and charge efficiency are
maximized according to a desired trade-off between the two
characteristics. Maximizing rate of charge transfer and charge efficiency
may alternatively be thought of as minimizing total time to balance
charge and total charge loss. A graph showing charge within each cell as
a function of time during the cell balancing process for an exemplary
battery pack is shown in FIG. 4. As shown in FIG. 4, implementation of
the quadratic program results in the charge within each cell moving
towards a common charge level without excessively decreasing the total
charge contained within the battery pack, thus balancing the charge
within the battery pack without excessively wasting energy.

[0034]In the second variation, the processor 130 may optimize for a
combination of characteristics, with the relative importance ascribed to
each characteristic differing based on the usage scenario. For example,
as mentioned above, the rate of charge transfer is preferably high when
the rate of energy transfer to and from the battery pack is high. In
these scenarios, the optimization calculations preferably put more weight
on maximizing rate of charge transfer (i.e., minimizing the total time to
balance charge) and less weight on maximizing charge transfer efficiency
(i.e., minimizing the total charge loss). To adjust weights of each
characteristic, adjustable variables are preferably integrated into the
optimization calculations. For example, in the variation that optimizes
using a quadratic program, variables used to represent the weight of each
characteristic may be integrated into the program and adjusted by the
processor 130 when the preferences change. The processor 130 preferably
detects when there is a high rate of energy transfer to and from the
battery pack and implements the adjustment to the optimization
calculations. Alternatively, the processor 130 may receive instructions
to apply such adjustments, for example, the user may input into the
processor 130 to prioritize for high rate of charge transfer and the
processor 130 may implement the adjustment based on the input from the
user. The user may provide an input that does not directly indicate an
optimization preference, but the processor 130 may determine an
optimization preference that best fits the input. This may be
particularly useful when the battery pack is applied to a mobile device
such as an electric vehicle and the user plugs the device into the
electrical grid for charging. The processor 130 then determines from this
user input that the optimization preference is for high charge transfer
rate and not high charge transfer efficiency. Alternatively, the
processor 130 may optimize equally for each characteristic. However, the
processor 130 may utilize any other optimization method.

[0035]In a third variation, the processor 130 selects the combination of
donor cells 114 and receiver cells 116 from a set of available
combinations of donor cells and receiver cells. In this variation, the
available combinations of donor cells and receiver cells may be
determined by the connection capabilities of the plurality of switches
140. Each of the available combinations of donor cells and receiver cells
is preferably assigned a score that is based on the resulting charge
transfer rate between cells, the resulting charge transfer efficiency
between cells, and the resulting charge balance of the battery pack. The
processor 130 then selects the combination of donor cells and receiver
cells based on the score; for example, the processor 130 may select the
combination with the highest score. Similar to the second variation, the
scores are preferably based on the usage scenario. For example,
combinations according to the second mode may be assigned higher scores
in usage scenarios with high rates of energy transfer to and from the
battery pack and combinations of the first mode may be assigned higher
scores in other usage scenarios. Alternatively, the processor 130 may
select the combination based on the score by selecting the combination
with the lowest score. Here, the combinations with the more desirable
results are scored lower than those with less desirable results. The user
may also adjust the scores based on user preference.

[0036]The charge balancing system 100 of the preferred embodiments may be
expanded to balance charge between more than one charge balancing circuit
connected to the plurality of cells 110, as shown in FIGS. 5 and 6. This
may be particularly useful where the plurality of cells 110 includes a
substantially high number of cells 110 that are connected in series.
Because the amount of charge that each capacitor 120 may hold is
relatively small relative to the charge held by each battery, the charge
balancing system 100 may benefit from having two charge balancing
circuits, each with a capacitor, to balance the charge within the battery
pack. Additionally, if a substantially high number of cells 110 is
connected to one charge balancing circuit, because of the increased
voltage potential from the high number of cells 110, the capacitor and
plurality of switches may need to be selected for much higher voltage
stand off ratings, which may increase the cost of the charge balancing
circuit. The battery pack may also include more than one string of cells
110 connected in series. In this variation, a charge balancing circuit
may be coupled to each string of cells 110. Each charge balancing circuit
is preferably coupled to a portion of the plurality of cells 110 and
preferably includes a capacitor, a plurality of switches, and a processor
that selects a combination of donor cells 114 and receiver cells 116 from
the portion of the plurality of cells 110, as shown in FIG. 6.
Alternatively, one processor may be used to determine donor cells 114 and
receiver cells 116 for each charge balancing circuit, as shown in FIG. 5.
The portions may be separate portions (shown in FIG. 5), but may
alternatively share individual cells (shown in FIG. 6). In a first
variation each charge balancing circuit may function to balance a portion
of cells 110 independently from another portion of cells 110. In a second
variation, the charge balancing circuits may cooperate to balance charge
between portions of cells 110, thus balancing the charge amongst the
entire string of cells 110. In this variation, charge balancing of the
string of cells 110 may include distributing charge within a portion of
cells 110 in a certain charge distribution profile. For example, in the
variation with overlapping portions as shown in FIG. 6, the portion of
cells 110 coupled to the second capacitor 120b and the second charge
balancing circuit may have lower overall charge than the portion of cells
110 coupled to the first capacitor 120a and the first charge balancing
circuit. To balance charge between the two portions, the first charge
balancing circuit may distribute additional charge from the first portion
into the overlapping cells 110, in other words, purposefully unbalancing
the charge within the first portion of cells 110. The additional charge
distributed to the overlapping cells is then transferred to other cells
110 within the second portion through the second charge balancing
circuit, thus balancing charge across the entire string of cells 110.
However, the cell balancing system too may be expanded in any other
suitable way.

2. Method of Balancing Charge

[0037]As shown in FIG. 7, the method S100 for balancing charge within a
battery pack that includes a plurality of cells that are connected in
series includes the steps of providing a capacitor S120, selecting a
combination of donor cells and receiver cells from the plurality of cells
in one of two modes: a first mode where the number of donor cells is
equal to the number of receiver cells and a second mode where the number
of donor cells is greater than the number of receiver cells S130,
electrically coupling the capacitor to the donor cells to charge the
capacitor S140, and electrically coupling the capacitor to the receiver
cells to discharge the capacitor S150. In the method of the preferred
embodiments, charge is moved between the plurality of cells through the
charging and discharging of the capacitor in Steps S140 and S150, and the
movement of charge between the donor cells and receiver cells preferably
balances the charge within the battery pack. The charge balancing method
S100 may also include sensing the charge in each of the plurality of
cells S110. In this variation, the step of selecting a combination of
donor cells and receiver cells from the plurality of cells also includes
the step of selecting a combination of donor cells and receiver cells
based on the sensed charge.

[0038]The step of selecting a combination of donor cells and receiver
cells preferably includes the steps of selecting a cell of a higher
voltage potential than the capacitor as a donor cell to charge the
capacitor and selecting a cell of a lower voltage potential than the
charged capacitor as a receiver cell to discharge the capacitor.
Alternatively, the step of selecting a combination of donor cells and
receiver cells may include the step of selecting cells connected in
series that have a combined voltage potential that is higher than the
capacitor as donor cells. In this variation, each of the donor cells may
have a voltage potential that is higher than that of the capacitor.
Alternatively, in this variation, a donor cell may have a voltage
potential that is lower than that of the capacitor. However, the step of
selecting a combination of donor cells and receiver cells may select
cells of any other suitable voltage potential relative to the capacitor.

[0039]A first variation of the step of selecting a combination of donor
cells and receiver cells from the plurality of cells S130 includes
selecting the combination of donor cells and receiver cells based on the
charge state of the cells. For example, a cell with a voltage potential
that is higher than the capacitor is selected as a donor cell and another
cell with a voltage potential that is lower than the donor cell and the
charged capacitor is selected as the receiver cell, thus transferring
charge between the donor cell and the receiver cell. In this variation,
the step of selecting a combination of donor cells and receiver cells may
include selecting a combination according to either the first or second
modes. To increase the rate of charge transfer between the donor cell and
the capacitor and, subsequently, the charged capacitor and the receiver
cell, the step of selecting a combination of donor cells and receiver
cells may include selecting a combination according to the second mode.
To increase the efficiency of charge transfer between cells, the step of
selecting a combination of donor cells and receiver cells may include
selecting a combination according to the first mode. However, any other
suitable combination of donor cells and receiver cells may be selected
based on charge.

[0040]A second variation of the step of selecting a combination of donor
cells and receiver cells from the plurality of cells includes the step of
selecting a combination that is optimized relative to a characteristic
selected from the resulting charge transfer rate between cells, the
resulting charge transfer efficiency between cells, and the resulting
charge balance of the battery pack. The step of selecting a combination
of donor cells and receiver cells may optimize relative to only one
characteristic. In a first example, the step of selecting a combination
of donor cells and receiver cells optimized relative to a characteristic
includes the step of selecting combinations according to the first mode
when optimizing for high charge transfer efficiency. In a second example,
the step of selecting a combination of donor cells and receiver cells
optimized relative to a characteristic includes the step of selecting
combinations according to the second mode when optimizing for fast charge
transfer rate. The step of selecting a combination of donor cells and
receiver cells may also optimize relative to each characteristic. This
type of optimization may result in selecting a combination of either the
first or the second modes. In this variation, the step of selecting a
combination of donor cells and receiver cells optimized relative to a
characteristic includes running optimization calculations. The step of
running optimization calculations preferably utilizes the sensed charge
of each of the cells. For example, the step of optimizing may include
running a quadratic program that determines a charge balance for the
cells in the battery pack that will maximize both the charge transfer
rate and the charge transfer efficiency between the plurality of cells.
The step of selecting a combination of donor cells and receiver cells
that is optimized for each characteristic may include optimizing each
characteristic equally, but may alternatively include optimizing each
characteristic at a different level. The different levels of optimization
may be based on usage scenario and/or user preference.

[0041]A third variation of the step of selecting a combination of donor
cells and receiver cells from the plurality of cells includes the step of
selecting a combination from a set of available combinations of donor
cells and receiver cells. The step of selecting a combination from a set
of available combinations of donor cells and receiver cells preferably
includes the steps of assigning each available combination with a score
that is based on the resulting charge transfer rate between cells, the
resulting charge transfer efficiency between cells, and the resulting
charge balance of the battery pack, and selecting a combination based on
the score, for example, selecting the combination with the highest score.
The step of assigning a score to each available combination preferably
also includes adjusting the score based on the usage scenario, for
example, increasing the score of a combination according to the second
mode when there is a high rate of energy transfer to and from the battery
pack and increasing combinations the score of a combination according to
the first mode in other usage scenarios. The step of adjusting the score
based on the usage scenario may also include adjusting the score based on
a user preference provided by the user.

[0042]While the step of selecting a combination of donor cells and
receiver cells from the plurality of cells of the battery pack is
preferably one of the above variations, any suitable combination of the
above variations or any other suitable method for selecting a combination
may be used.

[0043]As a person skilled in the art will recognize from the previous
detailed description and from the figures and claims, modifications and
changes can be made to the preferred embodiments of the invention without
departing from the scope of this invention defined in the following
claims.

Patent applications by Jessica Riley, Mountain View, CA US

Patent applications by Steven Diamond, San Mateo, CA US

Patent applications by Vishal Parikh, Palo Alto, CA US

Patent applications in class With discharge of cells or batteries

Patent applications in all subclasses With discharge of cells or batteries